1. Field of the Invention
The present invention relates to a valve driving apparatus for performing variable control of a lift amount, lift timing and an operation angle of a valve in accordance with an accelerator opening degree in an internal combustion engine in, for example, a motorcycle or an automobile.
2. Description of the Related Art
In this type of internal combustion engine, one having a valve driving mechanism including a three-dimensional cam for making the valve lift amount, the valve timing, and the valve actuated angle continuously variable, and a spherical cam follower in point contact with a cam surface provided at a valve lifter is known. See Japanese Laid-Open Patent Application No. Hei 4-187807 (JP '807). According to JP '807 the cam surface and the valve lifter are in point contact with each other, therefore the valve lifter is able to follow a complex cam surface shape.
In the conventional engine with the valve driving apparatus using such a three-dimensional cam, a shape of a cam top portion of an intake cam is formed to gradually increase the valve operation angle and the valve lift amount, in order to change an intake air amount to a cylinder from an idling status to a full opened status. Besides, an exhaust cam also has the shape to gradually increase the valve lift amount in accordance with the intake cam.
In this case, to perform an efficient gas exchange, the exhaust cam is required to have the valve operation angle about twice as large as that of the intake cam even when the intake cam is in the low lift status. To realize such a valve operation angle, the valve lift amount of the exhaust valve consequently should be twice or more as large as that of the intake valve considering the valve jump and the bounce (see a valve lift curve at the intake side and the exhaust side shown by a solid line in FIG. 13).
Here, a short summary of the valve driving apparatus having this type of three-dimensional cam is explained. For example, as shown in a FIG. 14, a roller state tappet is constantly in contact with a three-dimensional cam, which is slidably movable in an axial direction of a camshaft (both arrow X). And, a valve is driven to open and close via the tappet which moves vertically (both arrow Y).
However, the exhaust cam having the above-described characteristic suffers a very large reaction force from a valve spring, in a rotation region of an engine low load which is frequently used under a normal operation. Further, in a case when a cam top portion has a gradually increasing shape, only a specific portion of a tappet is in contact with the cam top portion (see a dotted circle portion in FIG. 14), and a biased abrasion will occur at the point of contact.